Extrusion devices are used to melt, blend, and form materials, such as plastics and polymers, into a desired shape. Typical extrusion devices include a rotating screw housed coaxially within a heated, cylindrically-shaped feed throat and barrel. A portion of the feed throat is cut away forming an opening for admission of materials. A hopper is coupled to the extrusion device for feeding the material through the opening, into the feed throat and subsequently into the barrel. The screw rotates within the feed throat and barrel and drives the material therethrough. The extrusion material is forced through a flanged discharge port at a discharge end of the barrel and into a downstream into a die or aperture.
One of the basic problems that has plagued polymer industry is that when the polymer exits too fast from an extruder or through flow restrictions, such as in valves, a surface of the extruded polymer starts to exhibit undulations or irregularities. The undulations become progressively stronger as the flowrate increases. At even higher flow rates, the distortions can become so severe that they cause the extrudate to break, which is referred to as melt fracture. Melt memory relates to the elasticity of the melted polymer and can also affect the quality of the extruded polymer.
As illustrated in FIG. 1, the prior art extrusion machine 10, includes a valve 100 mounted on a discharge flange 11 of the extrusion machine 10 via a coupling 113. The valve 100 includes a body 110 having an inlet 112 and an outlet 114. The outlet 114 is positioned proximate an opposing end 116 of the inlet 112 and is oriented at an angle of about ninety (90) degrees from the inlet 112. The opposing end 116 has an opening 118 therein and that is coaxial with the inlet 112 relative to a longitudinal axis L. A generally cylindrical plug 120 having a bore 121 extending therethrough is positioned in the opening 118. The plug 120 defines a flange 122 formed on a distal end thereof. The flange 122 is secured to the opposing end 116 of the valve 100 with suitable fasteners 123. The plug 120 also defines a body portion 124 that extends partially into an interior area 100A defined by the valve 100. The body portion 124 defines a radially inward tapered surface 126 that extends from one end 124A of the body portion 124 towards the flange 120 and terminates therebetween by transitioning into a constant diameter D defined by the bore 121. A screen pack 133 is positioned in the interior area 100A between the inlet 112 and the end 124A of the plug 120.
Referring to FIG. 1, a flow control member 130 extends into the bore 121 concentrically with the longitudinal axis L. The flow control member 130 defines a tapered end 132 that is positioned in the bore 121. The flow control member 130 translates along the longitudinal axis L in the bore 121 and a through packing gland 140. The flow control member 130 controls the flow of high pressure polymer material discharged from the extrusion machine 10, as described below. The flow control member 130 can be moved by a suitable operator, such as a hand-wheel. The packing gland 140 includes packing 142 which is compressed between walls of the bore 121 and an exterior surface 130E of the of the flow control member 130. The packing 142 is compressed by a packing follower 144 that is moveably secured to the end 116 of the valve 100 by suitable fasteners 145, for example packing gland bolts.
Referring to FIGS. 1 and 2, the outlet 114 defines a first opening 151 and a radially inward and axially outward tapered surface 152. The tapered surface 152 extends between the first opening 151 and a transition point 153 of constant diameter of an outlet port 154. There is a non-uniform flow area 160 defined at an intersection of the flow control member 130 and the general area defined by the outlet 114, for example, proximate the first opening 151.
During operation of the extrusion machine 10 and prior art valve 100 of FIGS. 1 and 2, high pressure extruded polymer is discharged into the inlet 112 at a high pressure (e.g., 2200 psi). The flow control device 130 is selectively positioned in the bore 121 and over the outlet 114 to control the flow of the polymer into the outlet 114 thereby reducing the pressure of the polymer to about 250 psi. As the polymer flows through the non-uniform flow area 160 creating areas of high local shear 160A, 160B, 160C and 160D. The areas of high local shear 160A, 160B, 160C and 160D can create melt fracture and melt memory effects which cause profile disturbances in the polymer.
In addition, movement of the control member 130 is difficult because of the high pressure in the interior area 100A which the flow control member 130 acts against.